Fix warning about number of mouse buttons

Change a warning to distinguish between too few buttons and too many.

Before this change:

$ xmodmap -e "pointer = 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15"
Warning: Only changing the first 15 of 10 buttons.
$

After this change:

$ xmodmap -e "pointer = 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15"
Warning: Not changing 5 extra buttons beyond 10.
$

Fixes #2 (closed).

Closes #2 (closed)